1. Field of the Invention
The present invention relates to an ink jet print cartridge, and, more particularly, to a bubble generator for an ink jet print cartridge.
2. Description of the Related Art
A typical ink jet print cartridge includes an ink reservoir and a printhead for controllably jetting ink onto a printing medium. The printhead uses a thermal mechanism for ejecting drops. Such a thermal type printhead includes a thin-film resistor that is heated to cause sudden vaporization of a small portion of the ink. The rapid expansion of the ink vapor forces a small amount of ink through an associated one of a number of nozzles in the printhead. Another type of printhead uses a piezoelectric mechanism for ejecting drops.
Conventional drop-on-demand printheads are effective for ejecting or xe2x80x9cpumpingxe2x80x9d ink drops from the ink reservoir, but require mechanisms for preventing ink from leaking through the printhead nozzles when the printhead is inactive. Accordingly, the fluid ink in the ink reservoir must be stored in a manner that provides a slight backpressure at the printhead to prevent ink leakage from the nozzles whenever the printhead is inactive. As used herein, the term xe2x80x9cbackpressurexe2x80x9d means the partial vacuum within the ink reservoir that resists the flow of ink through the printhead nozzles. Backpressure is considered in the positive sense so that an increase in backpressure represents an increase in the partial vacuum. Accordingly, backpressure is measured in positive terms, such as water column height.
The backpressure at the printhead must be at all times strong enough for preventing ink leakage, and yet must not be so strong that the printhead is unable to overcome the backpressure to eject ink drops. Accordingly, the ink jet print cartridge must be designed to operate properly despite environmental changes that cause fluctuations in the backpressure. Such environmental changes can include, for example, changes in ambient atmospheric pressure such as that caused by changes in altitude. Accordingly, the level of backpressure within the ink jet print cartridge must be regulated during times of ambient pressure change.
In addition to environmental effects, the backpressure within an ink reservoir is also subjected to xe2x80x9coperational effects.xe2x80x9d One significant operational effect occurs as the printhead is activated to eject ink drops. The depletion of ink from the ink reservoir increases (makes more negative) the reservoir backpressure. Without regulation of this backpressure increase, the ink jet printhead nozzles will eventually fail because the printhead will be unable to overcome the increased backpressure to eject ink drops.
One attempt to regulate ink reservoir backpressure in response to environmental changes and operational effects includes mechanisms commonly referred to as accumulators. One such mechanism provides an accumulator working volume that is sufficient for operating the nozzles notwithstanding extreme environmental changes and operational effects on the backpressure within the reservoir. The accumulator changes the overall volume of the reservoir, thereby to regulate backpressure level changes, so that the backpressure remains within an operating range that is suitable for preventing ink leakage while permitting the printhead to continue ejecting ink drops. For example, as the difference between ambient pressure and the backpressure within the nozzles decreases as a result of ambient air pressure drop, the accumulator moves to increase the reservoir volume, thereby to increase the backpressure to a level that prevents ink leakage. The accumulator also moves to decrease the ink reservoir volume whenever environmental changes or operational effects cause an increase in the backpressure. For example, the decreased reservoir volume attributable to accumulator movement reduces the backpressure to a level within the operating range, thereby permitting the printhead to continue ejecting ink. Even with an accumulator having a large working volume, there may be instances where the accumulator reaches its maximum working volume while an appreciable amount of ink remains in the reservoir. Continued printing to remove this remaining amount of ink could increase the backpressure by an amount outside the range for proper printhead operation, and in the event this occurs, printhead failure will also occur.
One approach used to solve this problem is to incorporate a xe2x80x9cbubble generatorxe2x80x9d in the ink jet print cartridge. A typical bubble generator is an orifice formed in the ink reservoir to allow fluid communication between the interior of the reservoir and the ambient atmosphere. The orifice is sized such that the capillarity of the ink normally retains a small quantity of ink in the orifice as a liquid seal. The geometry of the orifice is such that when the backpressure approaches the limit of the operating range of the printhead, the backpressure overcomes the capillarity of the ink and the liquid seal is broken. As a result, ambient air xe2x80x9cbubblesxe2x80x9d enter into the ink reservoir to reduce the backpressure so that the printhead can continue to operate. When the backpressure drops, ink from the reservoir reenters the orifice and reinstates the liquid seal.
One such bubble generator consists of a tubular boss and a sphere mounted concentrically within the boss. The outside diameter of the sphere is smaller than the inside diameter of the boss to define an annular orifice. The sphere is maintained within the boss by a number of raised ribs formed around the interior of the boss. In this manner, the sphere can be press fit into the boss and maintained in position by the ribs. The raised ribs are sized to provide the necessary interference for a press fit to maintain the sphere within the boss and provide the necessary clearance from the inside wall of the boss. The sphere serves as a capillary member to maintain a quantity of ink within the boss. As a result, even when the pen is oriented such that the boss is not submerged in ink in the reservoir ink, a quantity of ink is trapped within the boss. Due to the curved surface of the sphere, the gap between the exterior surface of the sphere and the inner wall of the boss is smallest at the orifice and increases as the distance from the orifice increases. This geometry, coupled with the capillarity of the ink, constantly urges the trapped quantity of ink toward the orifice, the smallest portion of the gap, to provide a robust seal.
Another such bubble generator employs a sphere that is loosely placed in a cone shaped tubular boss having a number of raised ribs formed around the interior of the boss. The sphere is held in place in the ribbed cone by a flexible plastic film positioned across the outlet end of the cone.
Both of the aforementioned bubble generator designs rely on tight dimensional control of the ribs to achieve the desired bubble admission pressure. For example, as the surface tension of the ink contained in the ink reservoir decreases, the dimensions of the capillary channels formed between the ribs and the sphere must be reduced to readjust the bubble admission pressure to a desired value. As a further example, an increase in ink reservoir elevation with respect to the printhead nozzles increases the column height of the liquid supported by the reservoir backpressure, and this increase in column height can only be maintained by increasing the reservoir backpressure, i.e., by increasing bubble admission pressure. This as well results in the need to reduce the dimensions of the capillary channels formed between the ribs and the sphere. In either case, these changes translate into a reduced rib height, which can result in a rib height that is difficult to maintain under manufacturing conditions.
What is needed in the art is an improved bubble generator for an ink jet print cartridge that overcomes the shortcomings set forth above by being simple in design, easily modified, and comparatively easy to manufacture.
The present invention provides an improved bubble generator for an ink jet print cartridge.
The invention, in one form thereof, relates to an ink reservoir having a bubble generator. The ink reservoir includes an enclosure defining an interior space and an exterior space. The interior space is adapted for containing a supply of ink. The enclosure has a passage formed therein which permits fluid communication between the interior space and the exterior space. The passage includes a surface, and the passage defines a first aperture and a second aperture, wherein the second aperture is adjacent the interior space. A sphere is positioned in the passage and contacts a portion of the surface of the passage. The surface of the passage has a shape that permits ink to pass between the sphere and the surface. A membrane is positioned over the first aperture to retain the sphere in the passage. The membrane includes at least one hole being sized to define a bubble admission pressure difference across a thickness of the membrane.
An advantage of the present invention is that it is simple in design.
Another advantage is that it can be easily modified to change the bubble admission pressure.
Yet another advantage is that it is comparatively easy to manufacture.